The present invention relates in general to a lithographic method for fabricating three-dimensional structures on the micrometric and submicrometric scale.
The method in question can be applied to the combined fabrication of ordered or single three-dimensional submicrometric structures comprising within themselves further lithographic structures which could not otherwise be inserted or produced by other lithographic processes. In particular, the latter can consist of regions of irregularities in periodic structures, such as point, line, surface or volume defects.
The method can also be used for forming structures suspended from other complex three-dimensional structures.
The method provides a tool for overcoming some of the present limitations of the production of three-dimensional structures based on the ordinary layer-by-layer lithographic processes which encounter difficulties in the generation of concave shapes, in the opening of voids within lithographic super-structures, or in the simple generation of suspended structures.
The technologies for fabricating complex three-dimensional nanostructures are particularly important in various fields of application, including: photonics, in which they are used for forming photonic crystals and corresponding interconnection networks for optical signals; optics, in which they are used for fabricating diffractive optical elements; microfluidics, in which they are used for fabricating elements for micro-devices for medical and biological use and micro-implants, and for the production of electromechanical and/or electrochemical microsensors and microsystems.
These are disciplines which require the development of highly complex devices in which the typical lateral topology of the layer-by-layer lithographic procedure is no longer adequate, and the development of complex three-dimensional micro- and nanometric structures is also required. Regions of irregularity, buried structures, cavities and concavities, and defects incorporated in regular volume structures, in particular line defects, can be used to form and bring into a functional state these devices and the structures of which they consist.
The following description will be focused on the technological aspect of the fabrication of photonic crystals and microfluidic mixers, providing a typical example of a complex structure which can be formed by the new lithographic method.
Photonic crystals are artificial structures with a periodic structure, which can wholly or partially absorb and/or confine electromagnetic radiation within a given frequency range. In terms of geometry, they can be period along one-, two- or three-dimensions. The last-mentioned type has the property of confining electromagnetic radiation in all directions, and therefore plays a key role in the transmission of light signals and of an electromagnetic wave in general.
However, effective transmission of the signal requires not only the presence of the three-dimensional photonic crystal, but also a network of interconnections, such as a waveguide network, filters, resonant cavities, and similar circuits for transmitting and/or processing electromagnetic signals, capable of carrying optical radiation within the 3D structure.
Other passive devices may take the form of optical signal filters, combiners and/or differentiators. The possibility of forming optical cavities also becomes essential for the fabrication of innovative active devices (in other words, those capable of generating light signals) such as lasers with low activation thresholds. What is required, therefore, is a technology for designing and forming line and volume defects within these crystals, and for positioning the defects precisely with respect to the periodic structure of the crystals.
At the present time, the fabrication of a 3D photonic crystal comprising waveguides or other active or passive optical circuit elements is one of the problems limiting the development of photonics.
In terms of methods, a distinction is made in the prior art between two categories of procedure for forming three-dimensional periodic nanostructures, particularly those incorporating internal defects. On the one hand, there are known sequential processes such as the aforementioned layer-by-layer or point-by-point processes, which permit precise control of the fabrication of the structure and consequently of the generation of defects within it. These processes are very inefficient and expensive, and therefore unsuitable for mass production, or even for research in some cases. On the other hand, there are known parallel processes (such as nanoparticle self-assembly, holographic lithography, and X-ray lithography processes) which intrinsically provide for three-dimensional fabrication and are therefore suitable for mass production, but cannot be used to form defects, and therefore, in the case in question, cannot be used to form an interconnection network for the transmission of light signals in a photonic crystal.
The two procedures are complementary to each other. Control of the detail permits the design of the waveguides, but not of the photonic crystal structure; conversely, the generation of volumes with a three-dimensional structure does not permit the control of the generation of defects within them.
U.S. Patent Application US2003/0002846 to Sigalas, under the title “Three dimensional photonic crystal waveguide apparatus”, describes a standard layer-by-layer fabrication method. Periodic structures which are obliged to develop within a plane are generated in each layer. The operation is repeated with alignments of the subsequent layers to form the three-dimensional structure. During the fabrication of a single layer, a single line defect is formed in the planar periodic structure, this defect being buried subsequently by the deposition of the subsequent layers.
The drawback of the layer-by-layer procedure described above is that it requires numerous lithography steps and process steps for the creation of the photonic crystal and of the defect incorporated in the volume of the crystal, and is therefore slow and expensive and thus unsuitable for mass production.
Parallel or self-assembly procedures have complementary properties. These procedures make use of chemical and/or physical phenomena (nanosphere packing, interference of coherent light beams, ion beam deposition, etc.) to generate intrinsically periodic 3D structures. The formation of the photonic crystal is usually completed by the infiltration of dielectric material with a high refractive index through the interconnected pores of the periodic structure which has been generated. The nature of parallel procedures is such that they cannot generate the anomaly required for the generation of a line or volume defect with controlled geometry.
X-ray lithography also belongs to the category of parallel procedures for forming photonic crystals. According to procedures described previously, the superimposition of a series of parallel beams originating from different directions enables a three-dimensional ordered periodic structure to be generated. In this case also, it is necessary to find an innovative method for introducing the defects into the photonic crystal.
To facilitate the understanding of a typical lithographic process for fabricating three-dimensional nanostructures according to the prior art, reference will be made to FIGS. 1a-1e which show schematically the steps of fabrication of a general photonic crystal structure. This relates to the case of X-ray lithography as an example of parallel lithographic process.
FIG. 1a shows an initial configuration comprising a substrate S on which a layer L of a resist R, for example a photosensitive or X-ray sensitive resin, has been deposited. In FIGS. 1b and 1c, the resist R is illuminated by a radiation X through a mask M. In this case, the resist is exposed in successive steps with different directions of incidence, in which the radiation strikes the structure at different angles. In the present example, a positive resist has been used, and therefore in the illuminated regions (forming the lattice G in the figure) the molecular bonds of the resist are broken, and the resist is removed by means of a suitable solvent.
In a subsequent step (FIG. 1d), a metal is electrolytically grown inside the hollow reticular structure G created in the unexposed volume of positive resist. The removal of the undeveloped volume of resist (FIG. 1e) produces the final desired three-dimensional metallic structure, in this case a periodic lattice of interlocking metallic columns supported by the substrate S. The step of infiltration of the metal can be replaced or followed by infiltration of dielectric materials by the sol-gel method, or by chemical deposition of dielectric materials from the vapour phase or liquid phase, in order to produce purely dielectric or combined metal-dielectric photonic crystals.
With reference to FIGS. 2a-2h, these show schematically, in sequence, a process for forming a defect within a periodic three-dimensional structure according to the described method.
Steps 2a-2c are identical to steps 1a-1c described previously. A first layer L1 of positive resist R is deposited on a substrate S, and the resist is then illuminated (by X-rays, for example) through a mask M in the desired directions. Thus a reticular structure G of un-polymerized resist, embedded in a volume of stable resist, is created in the body of the resist.
In step 2d, the structure thus obtained is exposed to a further lithographic process, for example electron beam lithography (a process which, as such requires no further masking), to form a surface region D of limited extent, representing the defect to be produced in the photonic crystal being fabricated.
In the next step (FIG. 2e), after removal of the un-polymerized resist, the electrolytic growth of metal is carried out within the reticular structure G.
In the next step (FIG. 2f), a second layer L2 of the same resist R is deposited on the intermediate structure thus formed, and a further step of X-ray lithography is carried out (FIG. 2g) in the same way as before (FIGS. 2b and 2c) to create a further reticular structure G′ of un-polymerized resist which is then removed.
The process is completed with the metallization of the hollow reticular structure G′ in the upper layer L2 of the total volume of resist, thus producing a three-dimensional photonic crystal structure incorporating the defect D within it (FIG. 2h).
Clearly, the defect can be a zero-dimensional, one-dimensional (line defect), two-dimensional or three-dimensional defect, depending on the surface region exposed to electron beam lithography.
This combination of X-ray lithography and electron beam lithography is generally advantageous. It combines the possibility of using a parallel production method (X-ray lithography) with the minimum of necessary layers, and therefore the minimum of necessary process steps, to introduce the defect. However, it has a number of drawbacks.
One of these is due to the fact that the volume of the final structure is obtained by superimposing two layers of the resist at whose interfaces the defect has been created, and the overall periodic structure forming the photonic crystal is produced at different times in these layers. This leads to an alignment problem among the upper periodic structure of the crystal and the lower structure, due to the difficulty of correctly aligning the masks used in the two steps of X-ray lithography, carried out before and after the formation of the defect, respectively. This problem arises whenever two, or more, distinct lithographic processes for the generation of the lattices in the two layers L1 and L2 are used.